High electron mobility transistor (HEMT), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a transistor incorporating a junction between two materials with different band gaps, i.e., a heterojunction, as the channel. The heterojunction can be contrasted with a doped layer that generally used by MOSFET. As used herein, the heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors or other materials. A commonly used material combination is GaAs with AlGaAs with the introduction of modulation doping for two-dimensional electron gas (2DEG) generation. Another used material combination is GAN with AlGaN with the introduction of polarization charge for 2DEG generation. The selection of the combination of the materials may vary in dependence on the application.
To allow conduction, semiconductors are doped with impurities, which donate mobile electrons (or holes). However, those electrons are slowed down through collisions with the impurities (dopants) used to generate them in the first place. HEMTs avoid this through the use of high mobility electrons generated using the heterojunction. The heterojunction allows creating a very thin layer of highly mobile conducting electrons with very high concentration, giving the channel very low resistivity, i.e., high electron mobility.
The HEMTs utilizing gallium nitride (GaN) HEMTs have high-power performance. As used herein, GaN materials that are suitable for transistors can include binary, tertiary, or quaternary materials.
FIG. 1 shows an example of a conventional GaN HEMT device, described in e.g., U.S. publication 2009/0146185, which could be designed to have a threshold voltage of −3V. Layer 10 is a substrate, such as of SiC, sapphire, Si, or GaN, layer 11 is a GaN buffer, and layer 12 is AlGaN, with 20% Al composition as an example (Al0.2Ga0.8N). Layers 11 and 12 are both Ga-face material. A negative gate voltage is required to deplete the 2DEG under the gate and thereby turn the device off.
The GaN HEMT device can include a Ill-nitride semiconductor body with at least two III-nitride layers formed thereon. The material which forms III-nitride layer 12, e.g., AlGaN, has a larger bandgap than that which forms buffer layer 11, e.g., GaN. The polarization field that results from the different materials in the adjacent III-nitride layers induces a conductive two dimensional electron gas (2DEG) region near the junction 9 of the two layers, specifically in the layer with the narrower band gap. The 2DEG region or channel is shown throughout the figures as a dashed line. One of the layers through which current is conducted is the channel layer. Herein, the narrower band gap layer in which the current carrying channel, or the 2DEG channel is located is referred to as the channel layer. The device also includes a gate electrode 18 and source and drain electrodes 16, 17 on either side of the gate electrode 18. The region between the gate and drain and the gate and source, which allows for current to be conducted through the device, is the access region 7. The region below the gate electrode 18 is the gate region 6.
The improvements in the design of GaN devices are focusing on single channel Ga-polar GaN based HEMTs. However, the conventional Ga-polar HEMT usually requires advanced process techniques such as the gate-recess structure, the F-treatment or capping, layers in making enhancement-mode (E-mode) device. Those techniques can deplete the 2DEG underneath the gate region but suffer from either controllability issue or lattice damage problem.
An N-polar GaN HEMT device has a reverse polarization field and can be advantageous over Ga-polar device in making single channel E-mode device with low access resistance, and in particular, for low voltage operation, see e.g., U.S. Pat. No. 7,948,011. However, despite the increased performance of the N-polar devices, the drive current under low voltage bias for N-polar GaN HEMT is smaller than the state-of-the-art Ga-polar GaN HEMT. This limitation of the drive current degrades the RF amplification capability and limits the output power density of the device.
Thus, there is a need for improvement in current drivability, output power performance and gate controllability of HEMT devices.